Operation of circuits having differential inputs requires balun transformers. At higher frequencies, the insertion loss of the transformers increases. Moreover, if a transformer is not used in a 50 Ohm system (i.e., requiring external matching due to non-standard transformation ratios) the transformer will exhibit a further degradation in insertion loss (IL=2 to 3 dbs in a non-50 Ohm environment). Combining these losses with the filter losses at its input, the overall loss of a 3-pole/Match/Transformer combination can easily approach IL=5 to 6 dbs. These losses in a receiver front end will have adverse effect. Furthermore, in the frequency range greater than 1 GHz, the use of transformers becomes impractical. Thus, a need exists for a coupling circuit that has differential inputs, outputs, eliminates the need to use a transformer, and provides the capability of n-poles of filtering.
Split ring resonator bandpass filters having a single-ended input port, and a differential-ended output are known. Referring to FIG. 1A, there is shown a two-pole split ring resonator bandpass filter having a single-ended input port, and a differential-ended output, taught in U.S. Pat. No. 5,017,897 to Ooi et al. However, that circuit may not be suitable for circumstances requiring more than two poles of filter selectivity.
Referring to FIG. 1B, there is shown a conventional filter transformer 50 having three interdigital resonators 52, 54, and 56, and a transformer 58. This approach provides a balanced output and three poles of filter selectivity, but suffers from the drawbacks discussed above.
Referring to FIG. 1C, there is shown a three-pole ring resonator 100 having three split-ring resonators, 102, 104, and 106. The capacitors C.sub.c1 and C.sub.c2 are coupling capacitors and C.sub.t1, C.sub.t2, and C.sub.t3 are coupled across the gap (or split) in the resonators to reduce their size while maintaining the electrical characteristics of a larger ring. This approach also provides three poles of filter selectively, but suffers from the following problems: (1) high insertion loss; (2) inadequate for balanced operation; and (3) large size.
Therefore, a need exists for a coupling circuit for circuits having differential inputs, that does not require a transformer and that provides more than two poles.